This invention is directed to blow molding systems, and more particularly to a blow molding system which preconditions preforms such that temperature distribution for each preform and among a plurality of preforms is uniformly distributed prior to reheating and blow molding.
One of the most critical process variables when blow molding some materials is the temperature of the preform as it is being blown. This variable often has a significant impact on the most important physical properties of the final blown article. It can also drastically affect productivity reducing the operating window of several other processing variables. Ideally the preform temperature should be as consistent as possible from one preform to the next in order to maximize production rates, maximize the final article's physical properties and minimize scrap. A temperature variation of up to 3.degree. C. will generally have no adverse effects on the process. There have been various previous attempts to minimize this variability. Several of the most relevant ones are discussed below.
U.S. Pat. No. 5,322,651 (hereinafter '651) addresses the adverse effects of variations in ambient air temperature during the thermal treatment of preforms immediately prior to blowing into the final article. The '651 apparatus mixes ambient air with the re-circulating air in the oven to maintain the temperature of the air passing through the oven and over the preforms at a constant level. It does not address the adverse effects of variation in the amount of heat contained in the preforms during the day as shown in FIG. 2 of the '651 patent. Since in prior art two stage processes, preforms are generally injection molded hours or days before they are finally blow molded into their final shape and they are stored for a substantial period of time, their temperature will equilibrate to that of the ambient storage temperature. From FIG. 2 of the '651 patent it is obvious that preforms entering the reheat/blow stage can differ in temperature by up to 11.degree. C. As described at the top of Column 2 of the '651 patent, prior art machines have used an infrared pyrometer to measure the surface temperature of the preforms. This measurement is then used to regulate the thermal processing of the preform. This apparatus claims to eliminate the need to adjust the infrared heat emitting sources in Column 4, lines 61 to 65. This is questionable since the amount of heat contained by the preform will vary during the day whereas the heat added to the preform will be constant since the heat emitting sources are adding the same amount of heat to each and every preform. This will result in varying heat content of the preforms exiting the conditioning ovens which is directly proportional to the ambient temperature variation during the day, producing product whose properties vary during the day.
U.S. Pat. No. 4,140,464 issued to Spurr et al. discloses an apparatus used to injection mold preforms. The preforms are removed at substantially their blowing temperature by a handling device and placed in a parison storage section where they are maintained at the blowing temperature until they are blown within the time frame of one injection molding cycle. Since the preforms are stored for a very short period of time, the inherent temperature variation from one preform to the next on removal from the injection mold will continue to be seen in preforms entering the blowing station.
U.S. Pat. No. 4,063,867 shows an apparatus with multiple sets of injection mold cores which support the preforms while they are moved from the injection molding station to a preblow station. The preforms are then moved to an oven where multiple sets of cores are stored for multiple injection molding cycles and finally to the blowing station. The preblown preforms are heated in an "oven" by blowing hot air over them while they are enclosed by an extension of the hot air supply duct. The purpose of the oven is to heat the preforms in preparation for the blowing station. The apparatus is assumed to equalize any variation in preform heat content since the preforms are "soaked" in the oven. This patent explicitly deals with uniformly heating individual preforms to the blowing temperature in an oven. It ignores the problem of variations in ambient temperature and the variation in heat content from one preform to the next, produced by each injection molding cycle.
U.S. Pat. No. 5,066,222 (hereinafter '222) to Roos et al. addresses the problems of uniformly heating a preform throughout its cross-section. It teaches heating the outside of the preform to a temperature below the blowing temperature; cooling the outside to prevent the outside from reaching a temperature above the blowing temperature while the inside wall continues to increase in temperature as a result of the heat added to and contained in the body of the preform from the first heating step; further heating the outside to a temperature above the blowing temperature, followed by a period where the outside and inside temperatures are allowed to converge to the desired blowing temperature. FIG. 2 of the '222 patent clearly shows that all the incoming preforms are at the initial temperature To.
The key to this method is the provision of time to allow the outside and inside temperatures of the preform to converge to a desired value by providing exterior cooling means for the preform while the inner surface continues to warm up. The teaching of this patent does not consider incoming preforms of varying temperatures or heat content. The assumption is that all incoming preforms are at the same temperature To. An incoming preform that is hotter or cooler than To will result in a curve similar to FIG. 2 of the '222 patent for that preform which is shifted by the difference between the actual incoming temperature of the preform and To. This means that the final temperature of the preforms entering the blow station will vary by this amount. As previously stated this difference can be up to 11.degree. C. making it very difficult or impossible to produce consistently acceptable quality blown articles.
U.S. Pat. No. 5,206,039 to Valyi discloses a preform conditioning apparatus with a conditioning section before the blow molding machine. The conditioning section can consist of more than one conditioning station. In a preferred embodiment, at least one cooling and one heating station are required. The disclosed apparatus has discrete conditioning stations into which the entire output of an injection molding machine cycle are simultaneously placed. The entire output is then picked up again and moved simultaneously to the next station. Another transfer device removes a portion of the preforms from the second conditioning station into the blow station. Each time that preforms are moved, they are gripped by their necks. This repeated gripping, picking up, moving and releasing adds complexity to the apparatus and increases the probability of damage to the neck finish. Since the preforms are removed in groups which consist of fewer preforms than the number placed in the last conditioning station, each group will undergo different conditioning. This adds another variable to the process that is very difficult to compensate for. This apparatus also does not address the issue of variation in the heat content of the preforms entering the conditioning process. This process does not have the ability to correct the inherent variability of the temperature of incoming preforms.
U.S. Pat. No. 5,326,258 discloses a means to heat temperature sensitive preforms prior to blow molding. In this method, preforms are introduced first into an equalization section where they are "uniformly equalized throughout". They are then transported by a continuously moving conveyor through subsequent heating/cooling, temperature equalizing, surface treatment and tempering stages/stations. There is no explanation provided regarding the initial equalization section. Further, the equalizing refers to ensuring that each individual preform has a uniform temperature throughout its cross-section. This equalization allows the heat energy contained in each individual preform to dissipate throughout its entire mass. It does not address the problem of "equalizing" preforms of differing heat content due to differential cooling in an injection mold or variable ambient temperatures. This patent does not teach how this equalization is done. The figure appears to provide inadequate means to achieve the equalization required to ensure uniform temperatures across all preforms and within each preform.
There exists therefore, a need for a blow molding system which includes a preconditioning section for insuring uniformity in temperature distribution among all preforms moved through the system.